Networks that primarily utilize data link layer devices are often referred to as layer two (L2) networks. A data link layer device is a device that operates within the second layer of the Open Systems Interconnection (OSI) reference model, i.e., the data link layer. One example of a data link layer device is a customer premises equipment (CPE) device, such as a switch, modem, Ethernet card, or wireless access point. Traditional L2 networks include Ethernet networks, Asynchronous Transfer Mode (ATM) networks, Frame Relay networks, networks using High Level Data Link Control (HDLC), Point-to-Point (PPP) connections, PPP sessions from Layer 2 Tunneling Protocol (L2TP) tunnels, and Virtual Local Area Networks (VLANs).
In some instances, a layer three (L3) network is used as an intermediate transport network between two or more L2 networks in order to allow communication between the L2 networks. In this type of configuration, the L3 network transparently transports L2 communications between the L2 networks, thereby allowing the L2 networks to share an L2 service. Common protocols for transporting the L2 communications through the intermediate L3 network are label switching protocols, such as Multi-protocol Label Switching (MPLS) protocols, Resource Reservation Protocol (RSVP) and the Label Distribution Protocol (LDP). In a typical environment, a source device, such as a router connected to one of the L2 networks, can request a path through the intermediate network. For example, with MPLS, this path is referred to as a Label Switched Path (LSP), and defines a distinct, dedicated, and guaranteed path through the network to carry MPLS packets from the source to the destination. The MPLS packets encapsulate the L2 communications, thereby effectively shielding the L3 network from the transported L2 information.
One example of an L2 service is the Virtual Private LAN Service (VPLS), also referred to as Point-to-multipoint (P2MP) L2 Virtual Private Networks (VPNs). In general, VPLS allows two or more remote customer networks to be transparently extended through the intermediate network as if the intermediate network does not exist from the perspectives of the customer networks. In particular, L2 communications, such as Ethernet packets, are transported between customer networks via the intermediate network. In a typical configuration, VPLS-enabled routers that are associated with the customer networks define LSPs within the intermediate network to carry encapsulated L2 communications as if these customer networks were directly attached to the same Local Area Network (LAN). To properly communicate via these LSPs, each of these VPLS-enabled routers store L2 information, such as Media Access Control (MAC) addresses, as well as VPLS information, such as local and remote VPLS site information. In this manner, these VPLS-enabled routers provide transparent L2 connectivity across the intermediate network and simulate a direct LAN.
The presence of a data loop may have severe adverse affects on a L2 network. For example, a data loop may result in consumption of significant bandwidth and resources. Thus, it is often necessary to eliminate data loops from the L2 network topology. One typical approach in addressing data loops is to deploy the Spanning Tree Protocol (STP) within the L2 network. In accordance with the STP, devices with the L2 network, such as Ethernet bridges, share information and eliminate loops by reducing the L2 network to a single spanning tree having a single path between end stations.
It is often undesirable, however, to utilize the STP to eliminate loops when one or more intermediate networks provide virtual private networks for remote customer networks. As one example, it is often undesirable to utilize the STP to communicate spanning tree information over the intermediate networks due to the high volume of information exchanged. Moreover, such STP implementations often require the intermediate networks to be “fully-meshed” with respect to STP. In other words, most if not all of the provider routers within the intermediate networks would need to support the STP.